This invention relates to paper web drying and in particular to the profiling of air in the drying process.
Yankee type hoods are among the main elements in paper web drying processes and a Yankee hood is an air distribution and drying system, which operates at high temperatures. Typically, a Yankee hood is shaped to be installed over and spaced from a portion of the circumferential surface of a rotatable cylinder. The drying air is heated and pressurized in the system and is then supplied to the Yankee hood dryer where it passes through nozzles at high velocity and impinges on the moving, drying web. The spent air is then collected in the dryer and returned to a recirculation system. Some of this spent air is exhausted, but the majority of it is recirculated to conserve heat.
The heat which is transferred from the impingement air from the nozzles to the paper is used to increase the temperature of the paper to its equilibrium drying temperature; evaporates the water from the paper; and increases the temperature of the paper above the equilibrium temperature after the surface water has been evaporated.
Increasing production rates called for in the industry today result in demands for higher and higher evaporation rates. Achieving evaporation rates considerably higher than those currently available must be realized largely through improvements to the Yankee hood system. In a Yankee hood, the evaporation is driven largely by convection heat transfer, brought upon by the effect of impinging jets of hot air and radiation heat transfer. Effectiveness of hood evaporation largely depends on geometry of impingement air, properties of impingement air, and temperature.
Uneven cross-machine direction temperature profiles that are directly related to the heat transfer or drying rate, are a major problem on many paper machines. Temperature profile problems can and often originate at the crescent header and nozzle box. This is more pronounced at higher operating temperature. Temperature profile problems can be caused in the dryer section by uneven condensate removal, uneven cross-machine direction moisture profile and uneven air distribution in the supply or exhaust. It can result in operational and quality problems including reel building, corrugated rolls, converting difficulties, and rejected papers. Many mills overdry the sheet to compensate for moisture profile problems. This results in higher energy consumption and reduced production.
The present invention addresses the problem of thermal non-uniformity in the drying section of the hood by providing a combination of elements that results in a more uniform temperature and a more uniform nozzle velocity in the cross-machine direction. This means that by having a thermal equalizer and a divider plate in the nozzle boxes, more uniform thermal profiles at certain distances in cross-machine direction are obtainable.
According to the present invention, there is provided a thermal equalizer for a paper web drying machine of the type that includes nozzle boxes fed by crescent headers. The equalizer is secured in a fixed location substantially centrally in the nozzle box adjacent a junction thereof with the crescent header and is spaced from adjacent surfaces of the nozzle box and header. The equalizer comprises a structure having a distorted diamond-shaped configuration in cross-section with upper and lower longitudinal center lines and having a pair of equal area, lower surfaces and a pair of equal area, contoured, concave upper surfaces for applying direction and substantive uniformity to air from the header that flows therefrom into the nozzle box, around the equalizer and out of the nozzle box onto the paper web.
The thermal equalizer of the present invention is useful in association with the paper drying element shown in Applicant""s U.S. Pat. No. 5,531,033 Control Profile Drying Hood; U.S. Pat. No. 5,784,804 Yankee Hood With Integral Air Heating System; U.S. Pat. No. 6,079,115 High Temperature Yankee Hood; and U.S. Pat. No. 6,094,838 Curl And Profile Correction With High Velocity Hoods.